So you're suggesting that
ultimate to just be a fatter/longer version of her E?
That sounds very linear.
Additionally, you're asking for
to have 2 knock ups.
Vice versa,
to have 2 roots.
Boy that sounds really linear.
But to make my comment less about how boring that would be.
Allow me to explain why they ults for each champ currently work as they do now:
As simple as it seems Maokai's ultimate is an engage tool and can be minorly used as a disengage option. The reason being it's outrageous reach. Meaning it can entrap an enemy team with little to no anticipation.
Zyra's on the other hand can be played in a moments notice when peel NEEDS to happen. It creates an immediate zone that forces the opponent in pursuit to either commit and face the knock up and empowered plant or peel back and avoid punishment.
You could make the argument of "Zyra having Maokai's ultimate can peel just as well" However because Maokai's current ultimate requires direction and range to take full effect. It's easier to whiff the roots in the wrong direction and end up having the enemy committing to their pursuit. Additionally it would act as a player morale boost knowing that the support whiffed their 'ultimate' form of peel.